Various composite sheets composed of a polyester film and a plastisol have been proposed as materials for electronic and electric parts. Various properties such as the adhesion of the polyester film with the plastisol, the electrical insulating properties, the low temperature flexibility and the non-migration of plasticizer of e.g. an ABS resin to the packaging material, are required for the composite sheets to be useful as materials for such electronic or electrical parts.
The present inventors have previously proposed in Japanese Patent Application No. 99561/1986 a plastisol composition with an object to provide a material which is suitable as e.g. a resist for a flexible printed circuit substrate and which has practical adhesiveness to e.g. a polyester film while maintaining excellent electrical insulating properties even when dipped in a household cleaning solution.
In the earlier invention, the object was accomplished by incorporating an epoxy resin and a blocked polyisocyanate to a plastisol composition. However, when such a plastisol composition is laminated on a polyester film, a reaction between the polyisocyanate and the epoxy takes place, and if the proportion of the epoxy resin is increased to improve the electrical insulating properties, there has been a problem that the adhesiveness to the polyester film tends to decrease.